Turbochargers with variable geometry, in general with adjustable guide blades, allow varying the ratio between exhaust gas throughput and turbine rotational speed and in this manner also providing a good compression performance even at low engine rotational speed. In order to achieve a required compression performance it can be necessary to set the guide blades so that the turbine runs near a permissible maximum rotational speed although the exhaust gas throughput is still below its maximum. A sudden increase of the exhaust gas throughput, particularly in connection with a sudden rotational speed increase when down shifting a transmission of the motor vehicle can then lead to the maximum permissible rotational speed of the turbine and being exceeded and thus the turbocharger put at risk. Mainly motor vehicles with automatic transmissions or automated manual transmissions are controllable to allow for very rapid downshifting operations which can result in such limit value being exceeded.
The problem is particularly pronounced when using the vehicle in thin air, i.e. when travelling at great altitude in mountains, since the reduced air pressure reduces the air mass sucked in by the compressor of the turbocharger per revolution and, accordingly, the work performed by the compressor on the sucked-in air per revolution is also reduced, so that the turbocharger is delayed to a lesser degree than when travelling under high air pressure in lowlands.
At least one object is to state a motor vehicle with a turbocharger. The risk of exceeding the maximum permissible rotational speed of the turbocharger is eliminated or at least reduced. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.